1. Field of the Invention
This invention relates to display devices, in general. In particular, the invention relates to alpha-numeric gas discharge displays which include a built-in-heater so that the display may be used in locations exposed to cold temperatures, for example, in gasoline pumps. More particularly, the heater is co-planar with the cathode electrodes, is easily adjusted for electrical resistance by selectively severing portions of the heater and is protected from inadvertent reconnection of the severed sections of the heater as a result of electrical shorting by condensed mercury, for example.
2. Prior Art
A typical gas discharge display device includes a chamber in which there is sealed an inert gas such as neon argon, or a mixture thereof. Of course, other inert gases can be used as desired. The gas in the chamber is, generally, maintained at sub-atmospheric pressure. Mercury vapor is often included within this chamber to impede ions of the gas from bombarding the cathode electrode while the display device is activated. However, mercury vapor tends to condense at about 0.degree. C. Thus, in applications where the gas discharge display device is cooled to temperatures at or below 0.degree. C., condensed mercury tends to settle onto the cathode electrode, as well as any other electrodes in this plane. The display device usually produces enough heat to maintain the mercury in the vaporized state during normal display operation. However, condensation of the mercury occurs when the display is turned off in a cold environment.
The condensed mercury tends to create a lump on the surface of the cathode electrode. The lump decreases the distance between the anode and cathode electrodes. This decrease in distance causes more current through that "hot spot" and produces a bright spot in the display. Furthermore, the increased current often causes the mercury to splatter throughout the chamber. When the mercury is splattered onto the anode electrode, a black spot can be created in the display.
Furthermore, it is possible for the mercury to solidify on the cathode electrode. A build-up of mercury can cause an arc discharge in the display device rather than the normal glow discharge.
Also, the condensation of mercury dictates that there will be less mercury vapor in the chamber to hinder the ion bombardment of the cathode electrode. The resultant increase in ion bombardment decreases the life span of the cathode electrode, thereby reducing the useful life of the display device.
Last, but not least the condensed mercury can also cause shorting between electrodes in the display device. For example, in some cases a selectively disconnected electrode is undesirably re-connected by the condensed mercury. As a result, certain characteristics of the display device are altered in an unexpected and uncontrolled fashion.
To eliminate these problems when the ambient temperature is below the condensation point of the mercury, it is necessary and/or desirable to heat the gas discharge display device prior to activation.
In the past, one approach has been to include an external heating unit behind (i.e. on the back of ) the display device. The external heating unit is normally connected to a separate power source. In cold weather, prior to switching on the gas discharge display device, the heating unit is turned on. However, in order to be efficient, the heating unit must be in intimate contact with the display and insulated from the surrounding environment to minimize heat loss. Also, where the gas discharge display is used in a gasoline pump, exposure to gasoline fumes can be undesirable for the heater unit because of the high temperatures and the possibility of combustion.
In another approach, the heater has been built into the display (see for example, U.S. Pat. No. 4,520,290; Cokefair). However, this approach requires a heater element disposed on an inner surface of the display chamber, an insulating layer disposed on the heater element, and a cathode layer disposed on the insulating layer. This approach has severe drawbacks in that at least two additional process steps are required in the production of the display device. The additional process steps also tend to alter the operating characteristics of the heater element which has been tailored for resistance levels prior to the process steps. Consequently, the heater operation is subject to uncontrolled modification. Moreover, because of the extra layer of insulation between the heater element and the cathode electrode layer, problems with cracking frequently occur as a result of the cyclic heating/cooling operations. Also, each successive layer increases the probability of an interelectrode short circuit developing. Likewise, pin-hole defects in the insulating layer are not easily detected when sandwiched between two conductors.
Another prior art device is taught by Person et al. in U.S. Pat. No. 4,692,655. This device uses a heater element which is printed on the same substrate surface of the display device as the cathode electrodes. The cathodes and the heater element can be printed at the same time. However, the heater element includes a plurality of "rung connectors" in a ladder-like conductor arrangement. Each of these "rung-connectors" includes a small break which can be selectively shorted by the subsequent application of a drop of conductor ink. This permits the selective alteration of the resistance of the heater element.
Unfortunately, this arrangement requires the heater element to be measured and finalized prior to a plurality of additional processing steps in the fabrication of the display device.
These steps include deposition steps, firing steps and the like which tend to alter the characteristics of the heater element after it has been finalized. Thus, the heater element is subject to a number of uncontrollable, unpredictable and irreversible alterations and variations of its operating characteristics. These alterations are of the type experienced with the Cokefair device noted supra.
Other shortcomings of prior art display devices are known to those skilled in the art. These shortcomings need to be overcome in order to produce a useful, efficient gas discharge display device capable of low temperature operation.